Families...who'd have them, eh?
I'm rather with Joko on this one; it sounds to me that you and your sister have very different personalities (my wife and my sister-in-law are like the proverbial chalk and cheese) and you have probably been putting out unconscious don't-want-to-be-a-bridesmaid signals for some time.
However, given the situation in your family at the moment and assuming an invitation to be a bridesmaid isn't forthcoming, it's probably best to keep silent about it, even though that might be a bit hard at the moment. Somebody might well say something about it, but it is by far the best if that person isn't you.